tools: psock_lib: harden socket filter used by psock tests

The filter added by sock_setfilter is intended to only permit
packets matching the pattern set up by create_payload(), but
we only check the ip_len, and a single test-character in
the IP packet to ensure this condition.

Harden the filter by adding additional constraints so that we only
permit UDP/IPv4 packets that meet the ip_len and test-character
requirements. Include the bpf_asm src as a comment, in case this
needs to be enhanced in the future

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>